With the advent of server virtualization, two basic assumptions of data center network design have changed. Firstly, multiple operating system (OS) images (or virtual machines) are now allowed to transparently share the same physical server and I/O devices and, secondly, the relationship between an OS image and a network is now dynamic. The access layer of the network may support local switching between different virtual machines within the same server, thus invalidating the traditional assumption that each network access port corresponds to a single physical server running a single image. Further, each virtual machine may be moved from one physical server to another within the data center or even across data centers.